Such a fluorescent lamp is known, for example from EP O 457 392 A1 wherein the luminescent layer comprises as its luminescent materials: (a) 33 to 44% by weight of a white luminescing alkaline earth metal halophosphate activated by trivalent antimony and bivalent manganese; 27% to 36% by weight of a blue luminescing alkaline earth metal halophosphate activated by trivalent antimony; and (c) 20 to 40% by weight of red luminescing yttrium oxide activated by trivalent europium wherein during operation of the lamp about one third of the total quantity of radiant energy supplied by the lamp is emitted in each of the three spectral ranges 380-490 nm, 490-590 nm, and 590-700 nm.
Another example of a known fluorescent lamp is that disclosed and claimed in EP 0 596 548 A1 in which the low pressure lamp comprises a combination of five phosphors including (a) a SAE or BAE (strontium or barium aluminate activated by bivalent europium) or a SBP (strontium borophosphate activated by bivalent europium); (b) a CBM (pentaborate activated by trivalent cerium and bivalent manganese)comprising Gd and Mg with red Mn.sup.+2 emission; (c) a BAM (barium magnesium aluminate activated by bivalent europium) or a SCAP (strontium chlorophosphate activated by bivalent europium; (d) a CAT (cerium magnesium aluminate activated by trivalent terbium) or a CBT (cerium magnesium pentaborate activated by trivalent terbium and trivalent cerium) or a CBTM (magnesium pentaborate comprising gadolinium and magnesium activated by trivalent cerium and by bivalent manganese and by trivalent terbium); and (e) a YOX (yttrium oxide activated by trivalent europium).
Such low pressure mercury vapor lamps, more commonly known as fluorescent lamps, have a lamp envelope with a filling of mercury and a rare gas and in which a gas discharge is maintained during lamp operation. The radiation emitted by the gas discharge is mainly in the ultraviolet region of the spectrum, with only a small portion being in the visible spectrum. The inner surface of the lamp envelope has a luminescent coating, often of a blend of phosphors, which emits visible light when impinged by the ultraviolet radiation.
Lighting accounts for approximately 20-25% of the electricity used annually in the U.S. For stores, offices and warehouses, lighting may account for up to 50% of their electrical consumption. Lighting is a primary focus of the National Energy Policy Act (EPACT) of 1992, which is specific federal legislation designed to drastically reduce national energy consumption. EPACT mandates energy efficiency standards for various lamp types in terms of lamp efficacy and color rendering index depending on the lamp size. Luminous efficacy is a measure of the useful light output in relation to the energy input to the lamp, in lumens per watt (LPW). The EPACT standards for fluorescent lamps are listed in Table 1 below. Fluorescent lamps which do not meet these minimum standards cannot be sold in the U.S. of America beginning with certain dates in 1994.
______________________________________ EPACT FLUORESCENT LAMP STANDARDS NOMINAL MINI- MINIMUM AVERAGE LAMP MUM LAW EFFICACY LAMP WATTAGE CRI (LPW) ______________________________________ 4-FOOT &gt;35 W 69 75.0 MEDIUM .ltoreq.35 W 45 75.0 BI-PIN 2-FOOT &gt;35 W 69 68.0 U-BENT .ltoreq.35 W 45 64.0 8-FOOT &gt;65 W 69 80.0 SLIMLINE .ltoreq.65 W 45 80.0 8-FOOT &gt;100 W 69 80.0 HIGH .ltoreq.100 W 45 80.0 OUTPUT ______________________________________
The legislation also provides for certain exemptions from the Act. For example, certain fluorescent lamps that exhibit a CRI of 82 or above are exempted and thus may be sold without reference to EPACT requirements for efficacy. Many fluorescent lamps on the market prior to 1994 do not meet these mandated minimum efficacy requirements and exhibit CRIs that are too low to qualify the lamps for exemption under EPACT. Such lamps include lamps of the types: F96T12 and F40T12 CW (cool white [4100K]), F96T12 and F40T12 (Daylite [500K]), and F96T12 and F40T12 (warm white [3000K]). Either the efficacy or the CRT of such lamps must be raised to either meet the EPACT standards or be exempt from them or it will no longer be possible to manufacture and sell such lamps legally in the U.S.
Luminous efficacy is dependent on the efficiency at which the phosphor coating converts the ultraviolet radiation from the gas discharge into visible light, on electrode losses (specifically the cathode drop), thermal losses and losses in the column gas discharge. Lamp electrodes have already been highly optimized by the lamp industry for specific applications. Losses in the column discharge are primarily a function of lamp geometry. Thus, for fluorescent lamps of a given length, diameter and lifetime, a further reduction of losses at the electrodes and in the discharge is extremely difficult to attain.
The fluorescent layer is the single most important contributing factor in determining the luminous efficacy of fluorescent lamps. Increasing lamp efficacy through development of new or improved phosphors, however, is very time consuming, as well as expensive. Research and development in phosphors is accomplished primarily through large numbers of experiments because results are not-always predictable theoretically. Substitution of known high performing phosphors for other, lower performing phosphors is often not feasible either. While certain phosphors, such as the so called narrow band, rare earth phosphors, typically yttrium oxide, produce high luminous efficacy and excellent color rendering in lamps intended for the high end of the market, they are less than optimum because they are too expensive for use in lamps intended for standard or low cost applications which presently employ the relatively low cost halophosphate phosphors. The halophosphates, while cheaper and often successfully used to reduce the quantity of the more expensive rare earth phosphors, are most often detrimental to color rendering properties of the rare earth phosphor because of inferior emissions in the visible spectrum. Typically, calcium halophosphate activated with antimony and manganese, for example, will have a color rendering index (CRI) of about 60 and lamps utilizing this phosphor will not be exempt from EPACT for this reason.
Furthermore, the thickness or coating weight of each type of phosphor has already been optimized for the greatest light output. In general, if a luminescent layer is too thin, some of the U.V. radiation from the discharge will pass through the layer without being converted into visible light. If the layer becomes too thick, however, the phosphor itself absorbs and/or reflects a portion of the visible light generated in the region of the layer near the discharge. It is therefore common to optimize the layer thickness for a satisfactory output. This has been determined to be about 30 to 40 microns in general. Even with this optimized layer thickness, however, the particular type of phosphor, the cost of the phosphor and emission properties thereof are still the primary considerations that affect the choice available to those desiring to produce fluorescent lamps.
Most fluorescent lamps with high CRIs use a mixture of narrow band phosphors to transform the U.V. mercury discharge into visible radiation. The red component in these lamps is typically yttrium oxide activated with europium. This phosphor is expensive and has a very narrow emission in the red area of the spectrum.
Full spectrum fluorescent lamps are also available. Such lamps use a variety of phosphor blends, with the red component being either yttrium oxide activated with europium, or other red emission phosphors, such as the very expensive magnesium fluorogermanate activated with manganese. Additionally, the typical phosphor blends used by the industry have a deficiency in red due to the narrow band emission of the yttrium oxide phosphor.